It relies on a lot of things. The base issue is that the pipelines (code segment that draws a part of the picture like space, HUD, cockpit, planet surface, sky, ...) do not release their work in the same color space (HDR, sRGB, RGB, undefined, ...). Some seem to apply a tone mapping to reduce the size of their color space, others don't. Some have their own gamma correction, others lose gamma data in the process,... Probably because each of them were done by a different team and these teams didn't communicate with each other.I do actually have Odyssey and play it - it looks different from Horizons for sure but we're back to the point about subjectivity aren't we? I don't have a problem with it, which is not the same thing as saying 'it looks identical to Horizons.' Because it doesn't. You want certain elements to be brighter and certain other elements to not be brighter - I get it but unless they introduce the ability to change individual elements of the scene its never going to happen.
So when it comes to merge all these results, depending on the monitor, how it's configured, calibrated, what type of connection is used (DVI, DP or HDMI), the graphics card and its settings, the intensions of the developers (so none in the case of FDev, no joke)... Dx tries to guess implicitly how to mix it all up correctly and falls more or less far from the plate depending on the case.
And finaly, add over the top of this fun mess, personnal perception and preferences.